This application is a continuation of application International PCT/JP00/05616 filed on Aug. 22, 2000.
The present invention relates to a porous preform vitrification apparatus for manufacturing an optical fiber preform by dehydrating, sintering, and glassifying a porous preform.
In order to obtain an optical fiber preform by dehydrating, sintering, and glassifying a porous preform manufactured by a VAD (vapor-phase axial deposition) process, an OVD (outside vapor deposition) process, or the like, generally the porous preform is heat treated in a predetermined atmospheric gas obtained by mixing chlorine, oxygen, carbon monoxide, etc. into helium (hereinafter referred to as a xe2x80x9ctreatment gasxe2x80x9d).
A porous preform vitrification apparatus used for the heat treatment of this porous preform comprises, as shown in FIG. 1, a furnace core tube 2 accommodating a porous preform 1 and a heating furnace 5 surrounding an outer circumference of the furnace core tube 2. A treatment gas controlled to a predetermined feed rate by a gas feed rate controlling means (Ma) 17 is introduced by an introduction pipe 3 from a lower portion of the furnace core tube 2. The exhaust gas is discharged into the atmosphere through an exhaust suction pump 31 linked to a gas discharge pipe 4 and an exhaust gas treatment device 32. A manometer (Pa) 11 and a pressure control valve (Ba) 15 are provided in the gas discharge pipe 4.
The exhaust gas treatment device 32 is for removing harmful gas such as the chlorine in the treatment gas fed to the furnace core tube.
A high temperature near 1400xc2x0 C. is required for the dehydration and sintering treatment of the porous preform, so carbon is usually used for the heating element 6 of the heating furnace 5. When the carbon is heated in the air at a high temperature, it is oxidized and consumed, therefore, in order to extend the service life of the carbon heating element, it is necessary to bring the interior of the heating furnace body to an inert gas atmosphere such as argon or nitrogen. The inert gas is introduced into the heating furnace body via a gas feed rate controlling means (Mb) 18.
Also, the furnace core tube 2 is generally made of quartz in order to maintain the purity of the porous preform, but when it is heated to a high temperature near 1400xc2x0 C., it easily softens and deforms, therefore it becomes necessary to balance pressures inside and outside the furnace core tube 2 to prevent the deformation of the furnace core tube 2 during the dehydration and sintering treatment of the porous preform.
For this purpose, the pressure in the furnace core tube is detected by the manometer (Pa) 11 and the pressure in the heating furnace body is detected by a manometer Pb 12, a detection signal thereof is introduced into a differential pressure detector 13, and this differential pressure signal is used to operate both of the control means of the pressure control valve 15 used for the furnace core tube exhaust gas and the gas feed rate controlling means (Mb) 18 used for the heating furnace body to balance the pressures inside and outside the furnace core tube.
In the dehydration and sintering treatment of a porous preform, however, it was found that it was not sufficient to balance the pressures inside and outside the furnace core tube and that it was also necessary to suppress to a minimum the pressure fluctuation of the treatment gas in the furnace core tube.
This is because, in the dehydration and sintering treatment of the porous preform, according to positional relationship with a heating zone in a longitudinal direction of the porous preform in the furnace core tube and a state of progress of the dehydration and sintering, it is necessary to change the heating temperature and the feed rate of the treatment gas of the atmosphere in the furnace core tube, but when a fluctuation occurs in the pressure of the treatment gas in the furnace core tube at this time, minute unsintered portions sometimes remain in the optical fiber preform obtained by the dehydration and sintering treatment.
Accordingly, in order to obtain a high quality optical fiber preform, it becomes necessary to minimize the pressure fluctuation of the treatment gas in the furnace core tube as much as possible.
Further, when the dehydration and sintering treatment is carried out in the treatment gas up to the end, the treatment gas remains inside the optical fiber preform. Therefore, in order to prevent that, it is necessary to switch the atmospheric gas in the furnace core tube from the treatment gas to nitrogen gas near the end of the dehydration and sintering treatment. Where a plurality of vitrification apparatuses are operated in parallel, however, it is also necessary to consider a means for preventing fluctuation of the pressure of the treatment gas in the furnace core tubes of vitrification apparatuses of other systems operated in parallel.
An object of the present invention is to provide a technique required for the manufacture of a high quality optical fiber preform as described above and not only to balance pressures inside and outside the furnace core tube of a porous preform vitrification apparatus, but also suppress the pressure fluctuations in the furnace core tube as much as possible.
According to a first aspect of the present invention, there is provided a porous preform vitrification apparatus provided with a furnace core tube accommodating a porous preform, a heating furnace surrounding the furnace core tube and heating the furnace core tube, a means for feeding a gas essentially consisting of helium to the furnace core tube, a feed rate controlling means, a discharging means, and a discharge rate controlling means, characterized in that a gas feed branch pipe is connected to the middle of the gas discharge pipe connecting the furnace core tube and an exhaust suction pump and in that nitrogen or air is fed from the gas feeding means provided at the front end of the gas feed branch pipe.
According to a second aspect of the present invention, there is provided a porous preform vitrification apparatus of the first aspect of the invention characterized in that a drain conduit is provided in the gas feed branch pipe connected from the gas feeding means to the gas discharge pipe.
According to a third aspect of the present invention, there is provided a porous preform vitrification apparatus according to the first aspect of the invention and the second aspect of the invention characterized in that provision is made of a mechanism for detecting a pressure difference between a pressure in a furnace core tube and a pressure in a heating furnace body and comprehensively controlling a feed rate of the gas to the furnace core tube, a discharge rate of an exhaust from the furnace core tube, a feed rate of an inert gas into the heating furnace body, a discharge rate of the gas from the interior of the heating furnace body, a feed rate of a gas such as nitrogen fed to the gas feed branch pipe, and further a gas discharge rate of the exhaust suction pump based on a differential pressure signal with the pressure in the furnace core tube as a reference.
According to a fourth aspect of the present invention, there is provided a porous preform vitrification apparatus of the first aspect of the invention to the third aspect of the invention characterized in that the feed rate of the nitrogen or air fed from a nitrogen or other gas feed branch pipe is controlled to 15 to 50% of the rate of the treatment gas essentially consisting of helium fed to the furnace core tube.
According to a fifth aspect of the present invention, there is provided a group of porous preform vitrification apparatuses comprised of a plurality of porous preform vitrification apparatuses according to the first aspect of the invention to the fourth aspect of the invention arranged in parallel, characterized in that an exhaust suction pump is provided for every porous preform vitrification apparatus and in that a common exhaust gas treatment device is provided on the discharge side of the exhaust suction pumps.
By employing the porous preform vitrification apparatuses according to the present invention, stable manufacture of a high quality optical fiber preform becomes possible.